1. Field of the Invention
The present invention relates to a molding method for making metal castings such as a housing of notebook computers or other electronic devices. The present invention also relates to a die used for implementing such a method.
2. Description of the Related Art
The housing of a mobile electronic device such as a notebook computer, a cellular phone or a PDA should meet several requirements. For instance, the housing should be strong enough to carry the incorporated components safely. Also, the housing should have high thermal conductivity for effective cooling of the incorporated components. Further, to be economical with resources, the housing should be made of a material that can be easily recycled. In light of these, the housing of a recent mobile electronic device is often made of metal rather than resin.
Mobile electronic devices, such as notebook computers and PDAs, need to be small in weight and size for convenience of carriage. Producing a lightweight device needs lightweight components. In a mobile electronic device, the metal housing may often occupy more than 30% of the gross weight, and thus it is important to make the housing lightweight for achieving the total weight reduction of the mobile device. Materials suitable for making such a lightweight housing are light metals, such as magnesium (Mg) and aluminum (Al), or light alloys whose main component is one of these light materials. Among the above-mentioned light metals, magnesium is very popular for producing a metal housing because of its high specific tensile strength, effective heat-dissipating nature (which rivals Al) and low specific gravity, which is about 70% of the specific gravity of aluminum.
As known in the art, various manufacturing methods, such as die-casting and thixo molding, can be employed to form metal housings of electronic devices. By these methods, however, a problem may occur in producing a thin-walled housing. Specifically, to provide a thin-walled housing, the die cavity should be narrow accordingly. Unfavorably, the narrow space of the die cavity may impede the otherwise smooth flow of the supplied molten metal. This is because the molten metal is cooled rather rapidly as it advances in the narrow cavity, and thereby the viscosity of the molten metal becomes unacceptably high before the supplied metal can fill the every part of the die cavity.
As a material for making a metal housing of a portable electronic device, Mg alloy such as AZ91D (9 wt % of aluminum, 1 wt % of zinc 90 wt % of magnesium) is widely used. This material, however, has rather poor fluidity since it was originally developed for forming large and thick-walled parts of an automobile. Therefore, when a thin-walled housing of a portable electronic device is made of such a Mg alloy, unfilled portions often result in the obtained casting. As for notebook computers of A4 and B5 sizes, the housings are expected to have a thickness of no greater than 1.0 mm and 0.7 mm, respectively. By the conventional molding methods, it is difficult to produce such a thin-walled housing from molten Mg alloy.
JP 2001-79645A discloses a method whereby a heat insulating member is provided in the cavity-defining surface for inhibiting thermal conduction from the molten metal to the molding die so that the fluidity of the molten metal is improved. The conventional insulating member, however, needs to be designed specially for the shape of the desired casting (and hence the shape of the die cavity). Due to this, the conventional method is rather costly and makes the resultant molted product expensive.